Page:The American Cyclopædia (1879) Volume XI.djvu/734

 716 MOLECULE are obliged to depend wholly on the combi- ning proportions for fixing their molecular weights ; but although in such cases we have various chemical means of controlling our re- sults, we are frequently in danger of assigning multiple values. Chemistry has shown that all matter is composed of one or more of 64 substances, which we call elementary sub- stances or chemical elements, because as yet they have not been decomposed ; and by the processes of chemical analysis we are able to determine with great accuracy the relative pro- portions of each element which a compound may contain. Moreover, as the masses are aggregates of perfectly similar molecules, it is evident that in analyzing a substance we an- alyze also its molecules; and that when we learn, for example, that water, in every hun- dred parts, contains 88-89 parts of oxygen and 11-11 parts of hydrogen, we know that every molecule of water must consist of the same elements in the same proportions. The follow- ing tables contain lists of volatile compounds of four elementary substances. Opposite to the name of each substance is, first, the weight of its molecule in microcriths, and secondly, the amount in each molecule of the elementary substance common to the list. The molecular weights are deduced primarily from the gas or vapor densities, but are corrected by the known combining proportions of the compounds. The amount of the elementary substance which each molecule contains is calculated from the results of chemical analysis. Atomic Weight of Carbon. Atomic Weight of Oxygen. NAMES OF COMPOUNDS OF OXYGEN. Weight of molwule. Weight of oxygen In molecule. Water 18 m.c. 28 " 80 46 74 44 46 64 60 80 104 146 208 ' 268-2 32 " 16 m.c. 16 " 16 " 16 " 16 " 82 " 82 " 82 " 48 " 48 " 48 " 48 " 64 " 64 " 82 " Carbonic oxide Nitric oxide Alcohol Ether Carbonic dioxide Nitric dioxide Sulphurous dioxide.. . Acetic acid Sulphuric trioxide Methylic borate Ethylic borate Ethylic silicate. Osmic tetroxide Oxygen gas Atomic Weight of Chlorine. NAMES OF COMPOUNDS OF CHLORINE. Weight of molecule. Weight of chlorine in molecule. Hydrochloric acid 86'5 m.C. 78-5 " 64-5 " 99 " 95 u 155-2 " 181-5 " 117-5 u 137-5 " 154 " 166 " 170 359-4 " 271-4 " 267-8 " 237 ^ 71 85'5 m.c. 35-5 " 85-5 " 71 71 71 " 106-5 " 106-5 " 106-5 " 142 " 142 142 177-5 " 177-5 " 218 218 71 Acetylic chloride Ethylic chloride Phosgene gas Dicarbonicdichloride.. . Chromic oxychloride. . . Arsenious chloride Boric chloride Phosphorous chloride. . . Carbonic tetrachloride. . Dicarbonic tetrachloride. Silicic chloride Tantalic chloride Columbic chloride Aluminic chloride Dicarbonic hexachloride. Chlorine gas An inspection of these tables will reveal one of the most remarkable facts in the whole range of physical science. The molecules of the compounds of any element always contain quantities of that element, which are simple whole multiples of a definite mass ; and this mass, which is the smallest quantity of an ele- ment found in the molecules of any of its compounds, is what we call the atom of that element. Thus the atoms of the four elements oxygen, chlorine, carbon, and hydrogen weigh respectively 16, 35'5, 12, and 1 microcriths respectively. It will be noticed that this defi- nition does not necessarily imply that the atoms are isolated masses like the molecules. When water is decomposed, two substances, which we call oxygen and hydrogen, are evolved from the aqueous mass, and from each mole- cule of water there must be evolved by the chemical processes the quantities of these ele- mentary substances which we call one atom of oxygen and two atoms of hydrogen. Whether now these masses preexisted in the molecule, or are formed from it by some unknown and unconceived transformation of its substance, is a question about which we can only specu- late. Nevertheless these atoms are definite and invariable quantities whether preexisting as isolated masses in the molecules or not ; and the only theory which has been advanced that gives an intelligible explanation of the facts NAMES OF COMPOUNDS OF CARBON. Weight of molecule. Weight of carbon In molecule. Marsh gas 16 m.c. 28 60 74 88 209 98 152 120 136 148 169 12 m.c. 24 86 43 60 72 84 96 108 120 ' 182 144 Oleflant gas Propylic alcohol Ether Amylic alcohol Triethylstibine Toluol Oil of wintergreen Cumole Oil of turpentine Amyl benzole Dipnenylamine Atomic Weight of Hydrogen. NAMES OF COMPOUNDS OF HYDROGEN. Weight of molecule. Weight of hydrogen in molecule. Hydrochloric acid Hydrobromic acid. . . 86-5 m.c. 81 128 27 18 84 81-5 46 17 84 78 60 28 16 46 74 2 " 1 m.c. 1 1 2 2 2 2 8 8 8 10 2 '* Hydriodic acid Hydrocyanic acid Water Hydric sulphide Hydric selenide Formic acid. Ammonia gas Hydric phosphide. . . . Hydric arsenide .... Acetic acid Oleflant gas Marsh gas Alcohol. . . Ether Hydrogen gas